As is known, many pourable food products, such as fruit juice, pasteurized or UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of this type of package is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by folding and sealing laminated strip packaging material.
The packaging material normally has a multilayer structure comprising a layer of fibrous material, e.g. paper, covered on both sides with layers of heat-seal plastic material, e.g. polyethylene.
In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material comprises a layer of barrier material, e.g. a sheet of aluminium or EVOH, which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material eventually forming the inner face of the package contacting the food product.
As is known, packages of this sort are produced, for instance, on fully automatic packaging machines, on which a continuous tube is formed from the web-fed packaging material. More specifically, the web of packaging material is unwound off a reel and fed through an aseptic chamber on the packaging machine, where it is sterilized, e.g. by applying a sterilizing agent such as hydrogen peroxide, which is subsequently evaporated by heating, and/or by subjecting the packaging material to radiation of appropriate wavelength and intensity; and the sterilized web is maintained in a closed, sterile environment, and is folded into a cylinder and sealed longitudinally to form a continuous tube in known manner.
The tube of packaging material, which in effect forms an extension of the aseptic chamber, is fed in a vertical direction, is filled with the sterilized or sterile-processed food product, and is fed through a sealing device to form the individual packages. That is, inside the sealing device, the tube is sealed at a number of equally spaced cross sections to form a continuous strip of pillow packs connected to one another by respective transverse sealing bands, i.e. extending perpendicularly to the travelling direction of the tube. The pillow packs are separated by cutting the relative transverse sealing bands, and are conveyed to a folding station where they are folded mechanically to form respective finished parallelepiped-shaped packages.
Packaging machines are known, as described for example in European Patent EP-B-0887265, which comprise two chain conveyors defining respective endless paths and fitted with respective numbers of sealing jaws. The two paths have respective branches substantially facing and parallel to each other, and between which the tube of packaging material is fed so that the jaws on one conveyor cooperate with corresponding jaws on the other conveyor along said branches of the respective paths, to grip the tube at a number of successive cross sections, and to seal and cut the packages.
Packaging machines are also known comprising only two pairs of jaws, which act alternately on the tube of packaging material to grip and seal, e.g. heat seal, it along a number of equally spaced cross sections.
As the sealing operation is completed, a cutter, carried, for example, by one of the jaws in each pair and interacting with the tube of packaging material, is activated to cut it along a centre line of the cross section just sealed, and so detach a pillow pack from the bottom end of the tube of packaging material. The bottom end being sealed transversely, the relative jaws, on reaching the bottom dead-centre position, can be opened to avoid interfering with the top portion of the tube. At the same time, the other pair of jaws, operated in exactly the same way, moves down from the top dead-centre position, and repeats the above grip/form, seal and cut process.
Alternatively, the packaging material may be cut into blanks, which are formed into packages on forming spindles, and the resulting packages are filled with the food product and sealed. One example of such a package is the so-called “gable-top” package commonly known by the trade name Tetra Rex (registered trademark).
More specifically, a succession of sheets of packaging material are fed to the machine and are folded to superimpose and seal the opposite edges in order to form respective substantially tubular or sleeve-shaped blanks open at both ends.
The tubular blanks are fed onto respective forming spindles movable along a given path; in particular, the spindles are commonly part of a conveyor, on which folding and sealing operations are performed.
More precisely, the tubular blanks are sealed at one end, filled with the pourable food product and then sealed at the opposite end to form a finished package.
In the above described types of packaging machines, the sealing operations are performed by gripping the packaging material between two pressure members and by supplying energy to heating means carried by one of the pressure members in order to obtain locally melt of the layers of heat-seal plastic material gripped between the pressure members.
In the case of jaw-type packaging machines, the pressure members are defined by the pairs of jaws acting on the tube of packaging material whilst, in the case of spindle-type packaging machines, the portion of packaging material to be sealed is generally gripped between one end of a relative spindle and a heating member movable towards and away from such spindle.
When the packages to be formed are of the aseptic-type with a layer of electrically conductive material, typically aluminium, as the barrier material, the sealing operations are normally performed by induction of electric loss current in the aluminium layer, so as to melt the heat-seal plastic material locally.
More specifically, one of the two pressure members between which the packaging material is gripped comprises a main body made of non-conductive material, and an inductor housed in a front seat on the main body; and the other pressure member has pressure pads made of flexible material, e.g. rubber.
The inductor is powered when the pair of pressure members grips the packaging material to seal a section thereof by heat sealing the plastic cover material.
In the case of packages with no layer of aluminium or other electrically conductive material, e.g. packages with an EVOH barrier layer, the sections of the packaging material are normally sealed using a hot bar to locally heat the packaging material inwards.
More specifically, one of the two pressure members, between which the packaging material is gripped, is fitted with the hot bar, while the other is normally fitted with one or more pressure pads of flexible material. The so-called “hot-bar” sealing method described calls for relatively prolonged contact between the hot bar and the packaging material.
To accelerate localized melting of the packaging material, and so increase package production speed, ultrasound sealing devices are widely used. These substantially comprise a mechanical-vibration generator, or sonotrode, and an anvil, e.g. as described in EP-B-615907, which are fitted to a respective pair of pressure members, and cooperate mutually to heat the packaging material by ultrasound vibration.
In all the above solutions, the heating or sealing means movable with the relative jaws or with the carrousel conveyor must be powered electrically by a fixed electric energy source at a predetermined point along the path of the heating means. In other words, electric energy must be transferred from a fixed source to a movable user device along a portion of the user device path.
This can be done using substantially two known methods, each of which, however, has drawbacks and limitations.
In a first method, the pressure members fitted with the heating means are also fitted with brushes preferably made of carbon, and which, along a predetermined portion of their travel, slide along respective copper power bars fixed to the packaging machine frame.
Rapid wear of the brushes and unstable contact between the brushes and the power bars are the main drawbacks of this method, which, moreover, tend to get worse as the output rate of the packaging machine increases.
To eliminate these drawbacks, over the past few years, a method has been perfected whereby electric energy is transferred from the fixed source to the movable user device by electromagnetic induction. One example of this method applied to chain-type packaging machines is described in Patent Application WO 00/64662, and a similar example, applied to a packaging machine featuring only one pair of jaws, is described in Patent EP-B-0732190.
In both cases, electric energy is transferred from a transmission unit, fixed to the packaging machine frame, to a receiving unit fitted to one of the jaws in a relative pair.
The transmission unit has a fixed magnetic core fitted with a primary winding connected to the electric energy source, and the receiving unit has a movable magnetic core integral with the body of the relative jaw and fitted with a secondary winding connected to the heating means. As the receiving unit travels past the transmission unit, electric current is induced in the secondary winding to power the heating means.
There being no contact between the receiving and transmission units, the above method solves the problem of wear.
On the other hand, the electric energy transfer systems by induction described in the above documents are inefficient and highly sensitive to variations in the air gap between the transmission and receiving unit cores. Since the size of the air gap depends on inevitable connection tolerances of the packaging machine component parts, the solutions described in the above documents fail to ensure, at present, the necessary reliability of the energy transfer system employed.
Patent Application WO 00/64662, in particular, employs an opposed E configuration for the fixed and movable cores. More specifically, each core comprises a bar portion, from which three parallel branches extend perpendicularly and, in use, face corresponding branches of the other core.
Tests conducted by the Applicant on this type of core configuration of the transmission and receiving units show a considerable increase in magnetic flux dispersion and in magnetizing current alongside small increases in the air gap between the fixed and movable cores.
In FIGS. 7 and 8, curves W3 and M3 show, for the opposed E configuration of the fixed and movable cores, apparent power and magnetizing current alongside an increase in the air gap between the cores. As can be seen, the curves are characterized by relatively high apparent power and magnetizing current values, and slope steeply, thus confirming the extreme sensitivity of the WO 00/64662 solution to variations in the air gap between the fixed and movable cores.